The present invention relates to an astigmatism generator appropriate for use in the pickup of an optical video disk player, a compact disk player, or for the like.
In an optical video disk player or the like, a focusing servo unit is provided in order to maintain an objective lens or the entire pickup at a prescribed distance, namely, at the focused position of the lens. Various systems have been proposed for generating a focusing error signal for operating the focusing servo. One such system is a so-called astigmatism system in which an astigmatism generator is disposed in the optical path of the lens. A typical astigmatism generator is a cylindrical lens 1 shown in FIG. 1. Since the cylindrical lens 1 generates only astigmatism and no other abberations, an accurate focusing error signal can be produced. However, since the surface of the lens needs to be cylindrically processed, the use of such a lens has the disadvantage that the manufacturing thereof is troublesome and expensive. Moreover, the cylindrical lens requires precise adjustment to make the center of the lens coincident with the optical axis.
In contrast with the cylindrical lens 1, mass production of a plane parallel plate shown in FIG. 2, which is an example of another astigmatism generator, is quite easy because the plate can be merely cut out of a plate of glass or the like which has plane parallel surfaces, making its cost very low. However, since the plane parallel plate 2 generates not only astigmatism but also comatic aberration, the distribution of light energy has a tendency to change from circular to triangular or the like so that the symmetry of the distribution is broken. This case is illustrated in FIG. 4. For this reason, it is difficult to produce an accurate focusing error signal when using a plane parallel plate.
Further, the use of a wedge-shaped plane plate 3, as shown in FIG. 3 has also been proposed to generate astigmatism. When the vertex angle .beta. of the wedge-shaped plane plate 3 is set at a prescribed value .beta..sub.0, coma can be eliminated. Since astigmatism is eliminated at another prescribed value .beta..sub.1 smaller than the former .beta..sub.0, a prescribed quantity of astigmatism can be generated by setting the vertex angle .beta. at the value .beta..sub.0, as shown in FIG. 5.
However, since it is necessary not only for the angle .beta. between two plane surfaces of the wedge-shaped plane plate 3 to be set to the prescribed value .beta..sub.0 but also for the distance (thickness t) between the two plane surfaces to be made precisely the same for all the wedge-shaped plane plates, there is a problem that mass production of an astigmatism generator in the form of a wedge-shaped plate is expensive.